The Digestive System

How the Body Works : The Digestive System
Starting at the mouth, the digestive system helps provide the energy your body needs to perform its many functions. Upon entry into a person's mouth, the teeth cut, tear, crush and grind food. In the mouth, salivary glands respond to the thought or presence of food by producing a fluid containing mucus and the enzymes amylase and maltase. The tongue then mixes the food and rolls it into a soft ball, called the bolus, which is pushed toward the esophagus. Passing through the esophagus the bolus is dropped into the stomach where gastric glands, one secreting digestive enzymes and the other secreting hydrochloric acid, begin to break the food down into smaller pieces. The stomach wall discharges mucus during this phase to protect itself against the action of the gastric acid. From there the food passes into the small intestine through the phylorus, a sphincter muscle that controls the flow of food. It is in the small intestine where a major part of digestion and absorption occurs. The intestinal wall releases enzymes which digest proteins, fats and carbohydrates. Blood and lymph vessels, which supply the small intestine, take away the final products of digestion. The lymphatics transport the fats around the body and finally release them into the bloodstream. Blood takes sugars and amino acids to the liver via the portal vein. The pancreas, like the small intestine, secretes enzymes in an alkaline juice to digest proteins, fats and carbohydrates. It also manufactures hormones which regulate the blood sugar level. From the small intestine, the digested food is received by the liver, which manufactures bile for the gall bladder. The gall bladder stores and discharges the bile, which helps to break down fats into minute droplets. Undigested food from the small intestine is then passed into the large intestine. There blood vessels supplying the large intestine carry away water extracted from the undigested waste. After passing through to the large intestine the ileocecal valve prevents digested food from returning to the small intestine. Undigested food is eliminated from the system through the anus.

Following Digestion of a Meal - Section 35.1
In this lecture, I first deal with the functions of the digestive system. We then take a journey through the digestive system by following your food from your mouth all the way through your body and even through the elimination of waste.
I also talk about enzymes like amylase in the saliva, pepsin in the stomach and bile from the liver and how those enzymes help in the process of digestion. So go ahead and join me on this journey through the digestive tract.

The Stages of Digestion
We get nutrients from the foods we eat through a process called digestion. Learn more about the stages of digestion and where in the body they take place.

How the Body Works : The Mouth
The mouth is the beginning of the digestive system. It is here that the first stages of food breakdown begin. When food enters the mouth, saliva, a fluid containing the digestive enzyme ptyalin and mucus, is secreted by the salivary glands. Saliva not only lubricates the food and begins its chemical digestion, but it also helps to keep the mouth cavity moist and clean. The teeth function mechanically to break up the food into smaller, more readily swallowed and digestible pieces. The tongue and the muscular walls of the mouth shape the food into a moist ball, the bolus, which is pushed to the back of the mouth and into the pharynx to be swallowed.

How the Body Works : The Throat
The throat is the pathway linking the mouth, nose, esophagus and trachea. When food is swallowed the soft palate closes off the postnasal cavity to prevent food from entering. At the same time a flap of elastic cartilage, the epiglottis, temporarily closes off the trachea to prevent food and liquid from entering.

How the Body Works : The Stomach
The stomach acts as a reservoir for food and has a capacity of two and a half pints. Within the stomach, solid food material is churned and kneaded for about three hours until it becomes a semiliquid mass known as chyme. The chyme is then forced into the small intestine, where the process of digestion is completed. The wall of the stomach has three muscular layers, an outer longitudinal layer, a middle circular layer and an inner oblique layer. As the stomach fills with food, wavelike contractions of the wall begin, and as these waves move along the stomach wall some of the food is passed through the relaxed muscle valve at the base and into the duodenum, the first part of the small intestine, where it is further digested before being absorbed into the body.

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